Symptoms
When you try to access a mailbox search in Exchange Control Panel (ECP), the Get-MailboxSearch task fails and returns the following error message:
Microsoft.Exchange.Diagnostics.BlockedDeserializeTypeException: Deserialization of type System.UnitySerializationHolder blocked due to NotInAllow at location MailboxDataStore exception.
Additionally, the mailboxes block the migration to Office365.
Resolution
To fix this issue, install the following security update:
